Process/reactor design heat transfer

Therefore, many traditional designs, such as stirred tank reactors, incorporate heat transfer in the process (jacket, external or internal coil, etc.). However, in these devices, there is a significant distance between the heat transfer site and the site of the chemical reaction where heat is released. As a consequence semibatch mode is implemented while batch mode and/or systems are diluted. 

In the second phase searches were made on the system and subsystem level. This is needed for the comparison of process alternatives and for the design of the exothermic reactor and its heat transfer systems. Carbonylation of methanol is an exothermic reaction. Thus only the exothermal reactors were searched. The CBR search found two cases which are general recommendations on the design of exothermic reactors with heat transfer systems. They are shown in Fig. 8 and 9. 

Designing reactor sorbers (increase and optimization of heat conductivity, maintenance of gas permeability, the coordination of processes of a heat transfer between heat carrier environments, the coordination of directions of heat and weights flows and so forth) ... 

Typical for strongly exothermic processes is that at some location in the reactor an extreme temperature occur, frequently named the hot spot. In some processes with very strong exothermic reactions the hot spot temperature can raise beyond permissible limits. This phenomenon is called runaway. An important task in reactor design and operation is thus to limit the hot spot and avoid excessive sensitivity of the reactor performance to variations in the temperature. The value of the temperature at the hot spot is determined mainly by the reaction rate sensitivity to changes in temperature, the heat of reaction potential of the process, and the heat transfer potential of the heat exchanger units employed. A heat exchanger is characterized by the heat transfer coefficient and heat transfer areas. 

The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heat-ing/cooling media. 

The tasks of chemical engineers are the design and operation of chemical reactors for converting specific feed material or reactants into marketable products. They must have knowledge of the rates of chemical reactions involved, the nature of the physical processes interacting with the chemical reactions, and conditions which affect the process. The rates of the physical processes (mass and heat transfer) involved in commonly used chemical reactors can often be estimated adequately from the properties of the reactants, the flow characteristics, and the configuration of the reaction vessel. Chemical process rate data for most industrially important reactions cannot, however, be estimated reliably from theory and must be determined experimentally. 

Moving from a laboratory-sized batch or semi-batch reactor means the heat transfer surface area to reaction volume ratio decreases. Most reactions or processes in laboratory-sized reactors are reaction rate limited, or, sometimes, diffusion rate limited. However, most commercialsized batch and semi-batch reactors are heat transfer rate limited. Therefore, we must determine the optimal ratio of heat transfer surface area to reactor volume when designing a batch or semi-batch reactor. 

With very exothermic reactions the number of beds would have to be uneconom-ically large to limit the temperature increase per bed. This problem has been solved by introducing the multi-tube reactor. A schematic illustration of a multi-tube reactor is shown in Fig. 11.3. A representative multi-tube reactor can contain hundreds or thousands of tubes with an inside diameter of a few centimeters [3]. The diameter is limited to this small size to avoid excessive temperature and hot spots. The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heating/cooling media. 

The interaction between chemical and coupled physical processes (mass and heat transfer) gives the tone to the progress of a chemical conversion in a reactor. Therefore, a prerequisite for designing a plant reactor is the knowledge of... 

The U.S. Department of Energy has funded a research program to develop the Hquid-phase methanol process (LPMEOH) (33). This process utilizes a catalyst such as copper—zinc oxide suspended in a hydrocarbon oil. The Hquid phase is used as a heat-transfer medium and allows the reaction to be conducted at higher conversions than conventional reactor designs. In addition, the use of the LPMEOH process allows the use of a coal-derived, CO-rich synthesis gas. Typical reactor conditions for this process are 3.5—6.3 MPa (35—60 atm) and 473—563 K (see Methanol). 

In the design of reactors for fluids in the presence of granular catalysts, account must be taken of heat transfer, pressure drop and contacting of the phases, and, in many cases, of provision for periodic or continuous regeneration of deteriorated catalyst. Several different lands of vessel configurations for continuous processing are in commercial use. Some reaciors with sohd catalysts are represented in Figs. 23-18 and 23-24. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

As is common in most polymer reactor design problems, heat transfer is one of the major process concerns. For example, if the heat transfer is primarily through the wall of a jacketed reactor, the overall heat transfer coefficient is a function of both the agitator configuration and the degree of swelling of the particles. 

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